The production of composite elements composed in particular of metallic outer layers and of a core composed of isocyanate-based foams, mostly polyurethane (PU) foams or polyisocyanurate (PIR) foams, is nowadays widely practiced on continuously operating twin-belt systems, and the elements are often also termed sandwich elements. Alongside sandwich elements for cold store insulation, elements with colored outer layers are of constantly increasing importance, these being intended for forming facades on a very wide variety of buildings. The outer layers used here comprise not only coated steel sheet but also stainless steel sheet, copper sheet or aluminum sheet. In particular in the case of facade elements, adhesion between foam and outer layer is decisive. If the color shade is dark, the insulated outward-facing outer layer can easily reach temperatures around 80° C. If adhesion of the foam to the outer layer is inadequate, blisters are produced on the surface, caused by separation of foam from the sheet, and these make the facade unattractive. In order to eliminate these problems, adhesion-promoter lacquers are applied before production of the coil is complete. However, reasons related to the process require that additives, such as flow aids, hydrophobicizing agents, de-aerating agents, and the like be present in the adhesion-promoter lacquers. These additives sometimes considerably impair the polyurethane foaming process. Added to this are interactions in the steel coil between lacquer on the outward-facing side and on the reverse side. The additional substances thus transferred to the reverse side mostly also have an adverse effect on the PU foaming process and lead to defects in the sandwich element. Even the corona treatment of the outer layers, which is now prior art, is in many cases inadequate to eliminate these defects. Furthermore, a very wide variety of reasons can prevent ideal adjustment of the twin-belt temperature for the particular system. This is particularly the case during production start-up procedures. This can likewise have an adverse effect on the foaming process and on the adhesion of the foam to the metallic outer layers.
Another frequent occurrence in the production of sandwich elements, caused by a wide variety of reasons, is undesired air inclusions, known as cavities, at the lower and upper outer layer, between sheet metal and rigid polyurethane foam. These air inclusions between sheet metal and foam can lead to blistering of the sheet metal particularly when severe temperature changes occur in the facade element application. Again, this then makes the facade unattractive.
Consequently, there is a requirement to find a process which gives a lasting improvement in the adhesion of the PU foams and PIR foams to metallic outer layers and which also withstands adverse external circumstances surrounding the production process. The process may be used continuously or batchwise. By way of example, a batchwise mode of operation can be used during twin-belt start-up procedures and for composite elements produced by means of presses which operate batchwise. Continuous use is required if the PU systems or PIR systems used intrinsically have very low adhesion to metallic outer layers.
At the same time, formation of cavities should be avoided in this process, in particular at the lower outer layer.
One possible way of improving adhesion is to apply an adhesion promoter to the outer layers. In the case of sandwich elements it is often the lower outer layer which has the poorest adhesion, determined in the tensile test. Furthermore, in conventional structures produced by means of sandwich elements the sheet-metal lower side is the outward-facing side of the facade and is therefore exposed to extreme conditions, such as temperature and suction effects, and is therefore subject to greater stress than the upper side of the sandwich element. For these reasons, it is possible to apply the adhesion promoter only to the lower outer layer. Once the adhesion promoter has been applied to the lower outer layer, the PU system or PIR system is applied to the outer layer, thus giving a composite element whose structure is: outer layer-adhesion promoter-rigid PU/PIR foam-outer layer.
There is a wide variety of established processes for the application of lacquers, adhesion promoters, adhesives, and thin films generally to sheet metal or to other substrates. Lacquers can be applied to substrates by means of dipping, spraying, electrostatic deposition, plasma coating, flow coating, or rolling. It is also possible to use spin coating to produce thin films on a substrate. Here, the substance is applied to the substrate and the substrate is then rotated, thus distributing the substance uniformly over the substrate. However, processes of this type are not practical in the case of the sheet metal used for production of sandwich elements.
A process very similar to spin coating likewise utilizes a rotating apparatus, but here the substance is ejected centrifugally and laterally via the rotation of the rotating plate. This technique is particularly good for internal coating of pipes or of other cavities, as described by way of example in U.S. Pat. No. 3,349,568, DE 2808903 and WO 9959730. One embodiment of this technique serves for the coating of moldings and also of sheet metal. However, in all of these processes the outer layers to be coated are conducted around the rotating plate and the substance is always applied to the relevant outer layer by centrifugal ejection from the rotating plate in a lateral direction, as described by way of example in DE 2412686. An electrical field is also sometimes applied here to improve application. However, all of these processes can form large amounts of aerosols which are hazardous to the environment and to health.